Question

The amount of which subatomic particle is different between an atom and its ion?

Answer 1

There are three subatomic particles known: (1) electron which is found outside the nucleus of an atom and (2 and 3) protons and neutrons which are both inside the nucleus. As they are outside the nucleus, it is easier to transport electron than any other subatomic particle. Thus, atom and its ion differ in the number of electrons.